[CRaC] Fix hangup after restoring

[YaSuenag]

I run following ApplicationRunner Spring Boot app and I obtained checkpoint by CRIU. The app did not finish after restoring.

  @Override
  public void run(ApplicationArguments args) throws Exception {
    if(args.containsOption("checkpoint")){
      System.out.println("Ready to obtain checkpoint...");
      // Wait restoring...
      cpCoordinator.await();
    }
    System.out.println("from Spring Boot App");
  }

I obtained thread dump, then I got following stack trace. It shows beforeCheckpoint CRaC handler waits signal in CyclicBarrier.

"prevent-shutdown" #29 [1504] prio=5 os_prio=0 cpu=0.17ms elapsed=25.76s tid=0x00007feb1017db00 nid=1504 waiting on condition  [0x00007feb4e22b000]
   java.lang.Thread.State: WAITING (parking)
        at jdk.internal.misc.Unsafe.park([email protected]/Native Method)
        - parking to wait for  <0x000000008a9279b0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park([email protected]/LockSupport.java:371)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionNode.block([email protected]/AbstractQueuedSynchronizer.java:519)
        at java.util.concurrent.ForkJoinPool.unmanagedBlock([email protected]/ForkJoinPool.java:3780)
        at java.util.concurrent.ForkJoinPool.managedBlock([email protected]/ForkJoinPool.java:3725)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await([email protected]/AbstractQueuedSynchronizer.java:1707)
        at java.util.concurrent.CyclicBarrier.dowait([email protected]/CyclicBarrier.java:236)
        at java.util.concurrent.CyclicBarrier.await([email protected]/CyclicBarrier.java:364)
        at org.springframework.context.support.DefaultLifecycleProcessor$CracResourceAdapter.awaitPreventShutdownBarrier(DefaultLifecycleProcessor.java:634)
        at org.springframework.context.support.DefaultLifecycleProcessor$CracResourceAdapter.lambda$beforeCheckpoint$0(DefaultLifecycleProcessor.java:606)
        at org.springframework.context.support.DefaultLifecycleProcessor$CracResourceAdapter$$Lambda/0x00007feb501c37c0.run(Unknown Source)
        at java.lang.Thread.runWith([email protected]/Thread.java:1596)
        at java.lang.Thread.run([email protected]/Thread.java:1583)

I investigated CracResourceAdapter, prevent-shutdown thread might through the second awaitPreventShutdownBarrier() call if that thread runs before awaitPreventShutdownBarrier() at beforeCheckpoint().

We need to separate barriers for beforeCheckpoint / afterRestore to work as expected.

[sdeleuze]

Could you please attach or share a link to the repository of your reproducer?

[YaSuenag]

Could you please attach or share a link to the repository of your reproducer?

Reproducer is here: https://github.com/YaSuenag/checkpointer/tree/main/example/springboot-cli

This is an example of checkpointer, library to implement CRaC event hooks and coordinate with CRIU events. This is not CRaC so event handling might be faster than CRaC, and it might surface race conditions which couldn't be see on CRaC.